The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the reduction of image artifacts caused by subject motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as xe2x80x9cspin-warpxe2x80x9d. The spin-warp technique is discussed in an article entitled xe2x80x9cSpin-Warp NMR Imaging and Applications to Human Whole-Body Imagingxe2x80x9d by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (xcex94Gy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
Subject motion remains a limiting factor in many MRI applications despite many suggested approaches to reduce or compensate for its effects. These include: altered phase encode (PE) ordering as described in U.S. Pat. Nos. 4,706,026 and 4,663,591; gradient moment nulling as described in U.S. Pat. No. 4,731,583; navigator echoes as described in U.S. Pat. No. 4,937,526; navigatorless acquisition trajectories that use the acquired data itself to track motion as described in U.S. Pat. Nos. 5,323,110 and 5,382,902; post processing techniques based on the raw data alone as described in Manduca, et al xe2x80x9cAutocorrection in MR Imaging: Adaptive Motion Correction Without Navigator Echoesxe2x80x9d, Radiology 2000; 215:904-909 and other phase correction strategies as described in Wood et al xe2x80x9cPlanar-Motion Correction Using K-space Data Acquired by Fourier MR Imagingxe2x80x9d, J. Magn. Reson. Imaging 1995; 5:57-64. It is well known that in-plane 2-D rigid body translations of an object during an MRI acquisition will create image artifacts along the phase encode direction in standard 2DFT imaging. Methods have been proposed to take advantage of the directionality of the artifacts to improve image quality by combining images with dissimilar artifact patterns. Two such methods are ghost interface techniques as described in Xiang et al xe2x80x9cTwo-Point Interface Method For Suppression Of Ghost Artifacts Due To Motionxe2x80x9d, J. Magn. Reson. Imaging 1993; 3:900-906 and the orthogonal correlation method as described in U.S. Pat. No. 5,729,140. Neither technique attempts to deduce the motion record or to correct the k-space phase errors.
The present invention is a method and apparatus for producing an MR image corrected for rigid body, in-plane, translational subject motion in which two separate acquisitions of k-space data are acquired for the same subject with the direction of the imaging gradients swapped. The phase differences between the two acquired k-space data sets at overlapping positions in k-space are then used to calculate corrections for each acquisition using a set of equations that describe phase changes caused by subject motion.
A general object of the invention is to correct for rigid body, in-plane, translational subject motion. By acquiring two data sets of the subject with the imaging gradient direction switched, sufficient information is contained in the k-space phase difference data to solve linear equations that describe phase corruption due to rigid body, in-plane translational motion. Corrective data is produced by solving these equations.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference is made therefore to the claims and herein for interpreting the scope of the invention.